Phenylalanine hydroxylase (PAH) is a multimeric hepatic enzyme that catalyzes the hydroxylation of L-phenylalanine (Phe) to L-tyrosine (Tyr) in the presence of iron, molecular oxygen and a redox-active cofactor, tetrahydrobiopterin (BH4). Phenylketonuria (PKU) is a heterogeneous group of disorders that can lead to intellectual disability, seizures, and impaired growth and development in affected children if left untreated with an average reported incidence rate of 1:12,000 in North America. PKU is a common in-born error of amino acid metabolism that is related to more than 500 disease-causing mutations of PAH or by a defect in the synthesis or regeneration of BH4. Due to the great allelic variation and large number of pathogenic mutations, universal newborn screening (NBS) for PKU relies on tandem mass spectrometry to detect hyperphenylalaninaemia in the population from dried blood spot extracts with follow-up diagnostic testing by quantitative analysis of plasma Phe and Tyr. Several PAH mutations have been shown to affect protein folding in the endoplasmic reticulum resulting in accelerated degradation and/or aggregation due to missense mutations (63%) and small deletions (13%) in protein structure that attenuates or largely abolishes enzyme catalytic activity. In general, three major phenotypic groups are classified in PKU based on Phe levels measured at diagnosis, dietary tolerance to Phe and potential responsiveness to BH4 therapy, including classical PKU (Phe>1200 μM), atypical or mild PKU (Phe is 600-1200 μM) and permanent mild hyperphenylalaninaemia (HPA, Phe<600 μM).
Currently, lifelong dietary restriction of Phe and BH4 supplementation are the only two available treatment options for PKU, where early therapeutic intervention is critical to ensure optimal clinical outcomes in affected infants. However, costly medication and special low-protein foods imposes a major burden on patients that can lead to malnutrition, psychosocial or neurocognitive complications notably when these products are not fully covered by private health insurance. Moreover, BH4 therapy is primarily effective for treatment of mild hyperphenylalaninaemia as related to defects in BH4 biosynthesis, whereas only 20-30% of patients with mild or classical PKU are responsive. Thus, there is an urgent need for new treatment modalities for PKU as an alternative to burdensome Phe-restriction diets, including large neutral amino acid formulations and enzyme replacement therapy using PEGylated recombinant phenylalanine ammonia lyase.
Thus, it would be desirable to develop an alternative method for the treatment of phenylketonuria.